This invention relates to a method and apparatus for exposing imaging sheets of the type described in U.S. Pat. Nos. 4,399,209 and 4,440,846 to The Mead Corporation, which are incorporated herein by reference. More particularly, it relates to a method and apparatus for exposing these imaging sheets by means of a light valve which functions as a wavelength or frequency transformer means.
U.S. Pat. Nos. 4,399,209 and 4,440,846 describe an imaging system wherein a photosensitive layer comprising microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation and subjected to a uniform rupturing force whereupon the microcapsules image-wise rupture and release the internal phase. The imaging system is particularly advantageous because it is a dry imaging system and does not rely upon the application of wet development processing solutions to develop the image. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules either in the internal phase or otherwise. When the microcapsules image-wise rupture, the released or activated color former reacts with a developer material and produces an image. U.S. Pat. No. 4,399,209 discloses a transfer system wherein the developer is provided on a separate sheet to which a transfer of the color former is made, and U.S. Pat. No. 4,440,846 discloses a self-contained system wherein the developer is on the surface of the imaging sheet with the microcapsules.
Presently, imaging materials prepared in accordance with the teachings of the referenced Mead patents are ultraviolet sensitive and tend to have absolute sensitivities in the range of about 100 to 10,000 ergs/cm.sup.2. These imaging sheets cannot be used to copy printed documents by reflection imaging using conventional radiation sources because they have very little sensitivity to visible light. A conventional ultraviolet source such as a fluorescent tube cannot be used to copy documents by reflection imaging because many documents are printed on papers which incorporate pigments, such as TiO.sub.2, which absorbs ultraviolet radiation. Thus, there is a need for a photocopy apparatus which is useful in imaging the Mead imaging material.